North America Weather and Climate Topics

DISCUSSION: On Monday, August 22, 2016, the National Weather Service (NWS) Boston/Taunton office confirmed that an EF-1 tornado touched down at around 3:20 a.m. E.D.T. near Concord, MA (About 20 miles northwest of Boston, MA in Middlesex County). The short-lived tornado developed maximum wind speeds of up to 100 mph with a path length of 0.5 miles and a maximum width of 400 yards. The tornado was associated with a larger system of storms that were fueled by an advancing cold front. The well-defined cold front that approached from the Ohio Valley had organized a marginally favorable thunderstorm environment across parts of the Northeast region. The predominant impacts across the Northeast included uprooted trees, torrential rainfall, brief flooding and damaging straight-line winds. NWS meteorologists successfully forecasted and confirmed the tornado by analyzing weather forecast maps, observing atmospheric conditions and assessing storm damages.

To illustrate, the Storm Prediction Center (SPC) surface-radar composite map below (3:00 a.m. E.D.T.), shows the direction of the winds behind and ahead of the cold front. Southerly winds (wind barbs pointing from the south direction) denote the transportation of the warm/moisture-laden air mass into the Northeast region ahead of the cold front (on the right-side of the precipitation system). The wind barbs that were oriented to the left of the precipitation system denote the presence of west-northwesterly flow, which accommodated the influx of cooler/drier air into the region.

Cold fronts undercut less dense (i.e. more buoyant, moisture-laden air) masses of warmer air that eventually evaporates and condenses into precipitation-producing clouds. As the cold front migrated across the region, the prominent system of storms had progressively moved into Massachusetts and the neighboring Northeast states. Therefore, the location of the cold front was along the western edge of the precipitation system. In this situation, cooler temperatures in the vicinity of 80°F displaced adjacent warmer air characterized by temperatures in the mid-80s. On Monday morning, the dew point temperatures dropped from 65°F at 5:00 a.m. to near 50°F by around 5:00 p.m. These favorable atmospheric conditions foreshadowed the convective outlook of potential severe storm activity.

In fact, according to NWS Meteorologist Joe DelliCarpini, NWS forecasters (as early as Saturday morning) anticipated the potential for an isolated tornado for Monday morning (after midnight). Per the Area Forecast Discussion on Saturday Aug. 20, 2016, forecasters indicated that a tornado could possibly develop along parts of the line of storms, which was expected to move along and ahead of the cold front. By this time, a developing low-level jet was transporting warm and moist air into the Northeast region. High humidity levels accompanied oppressive dew point values above 70°F and air temperatures were unseasonably warm.​Significantly, low-level jets are typically consumed of strong wind speeds located within about a mile above the ground. Therefore, low-level jets produce intense storm environments, which are often associated with wind shear. Two days later, at 3:20 a.m. E.D.T., storm rotation had developed within penetrating thunderstorms over parts of Middlesex County. Emergency alerts were issued at 3:01 a.m. E.D.T. and a tornado warning that lasted until 4 a.m. E.D.T. was issued at 3:24 a.m. E.D.T.

NWS Meteorologist Chris Legro and staff from the NWS Gray/Portland, ME office produced and modified an upper air proximity sounding near the time period at which the tornado occurred. By examining the sounding (attached below), a veering wind profile is noticeably relative to atmospheric altitude. Southeasterly winds near the surface transitioned to southwesterly flow with increasing height. Winds that veer with increasing height generate favorable wind shear profiles for tornadic storms.

Additionally, the aforementioned SPC mesoscale radar image showed a low-pressure system across Eastern Massachusetts. Middlesex County, which was near the low, was hit hardest (throughout the state of Massachusetts) by severe weather that morning. Storm survey reports entailed damaged rooftops and uprooted trees that fell in opposite directions due to gusty winds. Large uprooted trees that are chaotically blown down in convergent or crossed patterns can help confirm the validity of a ground-intact tornado.

While tornadoes rarely occur in Massachusetts, the emergency alert system proved to be very effective. Majority of the residents who were in the tornado warning area had a 20-minute lead-time after receiving emergency weather alerts on their wireless devices and NOAA weather radios. Fortunately, there were zero fatalities and the NWS Boston/Taunton office received an appraisal of appreciation on their Twitter and Facebook page. To learn more about other high-impact weather events, be sure to click here!